Re: error with pcap_findalldevs()

[Guy Harris <guy () alum mit edu>]

On Jan 15, 2004, at 10:35 AM, Mustafa Abu Sedera wrote:

> I had pcap 0.6.1 installed on my redhat 8. I wanted to use the 
> pcap_findalldevs funcion so i installed 0.8.1 on top of it (i don't 
> know how to uninstall a non rpm package, i am noob still).
>
> when I use the function I get an error: undefined reference to 
> pcap_findalldevs(). I checked the pcap.h file (in 2 locations 
> /usr/include and /usr/local/include) and found the function defined in 
> there.
>
> Strange is that:
> 1- the compiler doesn't complain about the definition of pcap_if_t 
> which i think wasn't available in 0.6.1

If pcap.h defines "pcap_if_t", why would the compiler complain?

> 2- when i enter an invalid type argument(eg: int in place of char* of 
> errBuf) the compiler displays: incorrect data type in argument of 
> pcap_findalldevs().
> And when I write 3 arguments it says: function takes 2 parameters.
> This would mean that the compilers "knows" the function.

If "pcap.h" declares "pcap_findalldevs()" with a prototype, the 
compiler *does* know the function.

However, it's the *linker* that's complaining about the undefined 
reference, not the *compiler*.  The compiler only looks at "pcap.h"; 
the linker looks at "libpcap.a" or "libpcap.so".

You probably still have libpcap 0.6.1's library installed, as well as 
having 0.8.1's library installed.  (You might also have both header 
files installed - is the "pcap.h" in "/usr/include" the same as the one 
in "/usr/local/include"?  If not, you do have both installed.)

There's probably one in "/usr/lib" and one in "/usr/local/lib".

You have to link with the 0.8.1 one, which is probably the one in 
"/usr/local/lib".

This means that, when you link your program, you need to have the option

        -L /usr/local/lib

in the compiler command line *before* the "-lpcap" option.

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